The present invention relates to an image forming apparatus including a thermal transfer printer for additively entering specific information, such as characters, that an original image does not include onto paper on which an image corresponding to an original image has been transferred and formed.
In some cases, it is necessary to make notes of such items as copy date, copier's name, and so forth on the paper on which an image forming apparatus, such as an electrostatic copying machine, has formed an image.
In such cases, it is troublesome to enter such notes by handwriting or stamping. There is known an image forming apparatus in which, to eliminate such troubles, such operation is automatically performed inside the body of the apparatus.
Such image forming apparatus is provided with a thermal transfer printer. In the thermal transfer printer, a casing, in which a cassette housing an ink-ribbon and a thermal head are attached, is supported by a pair of frame side plates, and a thermal head applies pressure through the ink-ribbon to paper passing a platen roller, thereby to heat and transfer characters and such onto the paper. Heat-meltable ink is on the ink-ribbon, and the ink is transferred to the paper with heat generated by the thermal head.
In such an image forming apparatus, however, a cassette whose ink-ribbon is used up is removed for replacement through a hole provided at the center of either of the frame side plates by a hand inserted through the hole after sliding the cassette in the axial direction of the platen roller.
In such an image forming apparatus it is difficult to replace a used ink-ribbon with a new one because the hole through which the used ink-ribbon must be replaced is narrow.
Furthermore, that the cassette is mounted in the vicinity of the thermal head in the casing makes the replacement of the cassette even more difficult. At the same time, there is a problem that a paper jam that may occur near the thermal transfer printer is difficult to clear.
Because the ink-ribbon is always placed near a passage through which hot paper passes, the ink-ribbon in such a thermal transfer printer rapidly deteriorates due to the heat and the like, although thermal transferring is performed only when it is necessary. In this connection, there is known a thermal transfer printer in which thermal transfer is performed while the thermal head is extending the ink-ribbon between a feed reel and a winding reel of the cassette, and after completion of the thermal transfer, slack of the ink-ribbon, stemming from the retraction of the thermal head, is eliminated, thus keeping the ink-ribbon away from the hot paper passage for the purpose of preventing such deterioration of the ribbon (see, for example, Japanese Laid-open Patent Application No. 297085/1988).
Meanwhile in thermal transferring, frictional force between the paper and the ink-ribbon is sufficiently greater than frictional force between the ink-ribbon and the thermal head (see, for example, Japanese Laid-open Patent Application No. 212977/1983). Therefore there is no relative shifting of the paper and the ink-ribbon. Accordingly, the ink-ribbon is wound from the feed reel by the length the thermal head travels.
There has not been proposed so far any invention or device wherein the length of the ink-ribbon to be wound is precisely regulated when winding the ink-ribbon in thermal transfer operations or when winding the slackened ink-ribbon after completion of thermal transfer.
When a relatively new ink-ribbon is drawn out from a feed reel and wound on a winding reel, the winding reel with the ribbon wound thereon has a smaller diameter than the feed reel with the ribbon wound thereon. On the other hand, when the ink-ribbon comes near to the end of its use, the winding reel with the ribbon wound thereon has a larger diameter than the feed reel with the ribbon wound thereon. Accordingly, if the winding reel rotates for a given period of time through a predetermined angle, the amount of the ribbon wound on the winding reel in the beginning differs considerably from the amount of the ribbon wound on the reel toward the end of its use. In other words, when the ink-ribbon is relatively new, the feed reel with the ribbon wound thereon has a larger diameter, and as the ink-ribbon is used, the diameter of the feed reel with the ribbon wound thereon becomes smaller.
The ink-ribbon is wound on the winding reel in thermal transferring, or the slackened ink-ribbon after thermal transferring is wound on the winding reel or the feed reel. In such cases, the amount of rotation of the reel to wind the ribbon on the reel differs depending on whether the ink-ribbon is new or has come near to the end of its use. If the amount of time the ribbon winds on the reel is adjusted for the case of a new ink-ribbon, there occurs the problem that as the ribbon draws near to the end of its use, the winding length onto the winding reel increases so that excess ribbon is wound on the winding reel. Also when the slackened ribbon is wound on the winding reel or the feed reel, excess ribbon is wound on the reel, or the winding amount is not sufficient to eliminate the ribbon slack.
In a thermal transfer printer as disclosed in Japanese Laid-open Patent Application No. 297085/1988, when the winding reel stops winding ribbon thereon, a tension spring urges the feed reel to turn by a predetermined angle in the direction opposite to the winding direction, thereby eliminating the ribbon-slack by winding the ribbon on the feed reel.
That thermal transfer printer, however, still suffers the same problem as described above in that the ribbon slack cannot be eliminated, even though the feed reel is turned by a predetermined angle in the opposite direction. If the amount of ribbon wound on the feed reel is small, the feed reel does not have a great enough diameter to wind the ribbon by a sufficient length to eliminate the slack.
In order to ensure the elimination of the ribbon slack even when the feed reel has a small amount of ribbon wound thereon, as mentioned above, the tension spring is adapted to urge the feed reel to turn by a larger angle.
In this structure, however, if the feed reel has a large amount of ribbon wound thereon, the ink-ribbon is always subject to tension after the slack is eliminated. This tension causes deterioration of the ink-ribbon.
In addition, because thermal transfer is performed with pressure of the ink-ribbon against the paper which is still very hot immediately after a fixing device has heated and fixed an image thereon, and because the ambient temperature around the thermal transferring device is very high, such image forming apparatus suffers the problem that the desired thermal transfer cannot be achieved because of the ink-ribbon melting in other locations than the pattern to be heated and transferred.
To cope with the above problem, it may be suggested that the paper discharged from the fixing device be exposed to air from a fan, thereby to cool the paper. However, if a fan is operated in the vicinity of the fixing device, the fan circulates air which has become hot due to the heat of the fixing device within the space between the pair of frame side plates, reducing the efficiency of cooling down the paper. At the same time, the fixing device is deprived of its heat for fixing, resulting in poor heating and fixing of an image.